For the manufacture of relief printing plates, particularly seamless resin plates for flexographic printing, using a photosensitive resin, the following methods are known.
(i) The method comprising coating a cylindrical support element externally with a photosensitive resin and subjecting the resulting photosensitive resin layer to imagewise exposure to obtain a relief image. By way of illustration, a cured polymer sheet of predetermined thickness (for example, about 1.5 mm) is bonded to the surface of a steel cylinder with an adhesive, then a curable photosensitive resin is coated in a predetermined thickness (for example, about 1.0 mm) on top of said cured polymer sheet, and finally the coating layer is imagewise exposed to obtain a relief pattern.
(ii) The method which comprises wrapping a photosensitive resin sheet around a cylindrical supporting element, welding the joint under heat and pressure, and exposing the sheet imagewise to obtain a relief image. A typical procedure comprises wrapping an unexposed photosensitive resin sheet accurately on the circumferential surface of a cylinder, wrapping a heat-shrinkable film on top of said resin sheet, heating the assembly under reduced pressure to soften the sheet and integrate the seam evenly with the remainder of the sheet while calendering the same during heating to prepare a smooth surface and finally exposing the sheet imagewise to give a relief image.
(iii) The method which comprises filling a clearance between an outer tube and an inner tube member of a double tube with a liquid photosensitive resin composition, and after curing of the resin, exposing the cured resin layer imagewise to give a relief image.
The method (i) has the following disadvantages. When a cured photosensitive resin sheet is bonded to the cylinder, there is formed a seam so that when a curable photosensitive resin composition is coated on top of the cured resin sheet, a variation takes place in coating thickness or coverage. In order to build up a sufficiently thick layer of such curable photosensitive resin, coating and drying must be repeated in multiple cycles but the formation of the above joint complicates the procedure and it is not easy to achieve a uniform coating thickness.
The method (ii) has the following disadvantages. It is difficult, even for a skilled worker, to wrap an unexposed photosensitive resin sheet around the supporting cylinder with perfect end alignment. Moreover, heating is required for evening the seam, which may cause degradation of the unexposed photosensitive sheet. Furthermore, as the seam cannot be evened in the strict sense of the term, the problem remains when it is desired to obtain a circumferentially seamless delicate image.
The method (iii) has the disadvantage that close dimensional tolerances must be respected in the fabrication of a double tube in order to assure a uniform layer thickness. Another serious drawback of this method is that air cells may enter the product cylindrical photosensitive resin sheet.
Furthermore, since the above-mentioned methods (i), (ii) and (iii) invariably involve the two steps of operation/i.e. formation of a photosensitive layer and subsequent exposure, the whole manufacturing process for printing plates is complicated.
Under the circumstances, the present inventor endeavored to find a commercially advantageous method for manufacturing a uniform-thickness, seamless cylindrical photosensitive resin structure and a commercially advantageous method for manufacturing a seamless cylindrical printing plate in one operation. The endeavor has resulted in the present invention.